A rare tumor in a 19-year old man caused hair to grow on his eyeball, researchers report.

The tumor, called a limbal dermoid, was benign and had been present since birth. It gradually grew in size until it was about 5 mm in diameter (a little less than a quarter of an inch), and sprouted several black hairs, said the researchers from Tabriz University of Medical Sciences in Iran.

Limbal dermoids are uncommon — an eye doctor may see just one or two cases during his career, said Dr. Mark Fromer, director of Fromer Eye Centers in New York City and an ophthalmologist at Lenox Hill Hospital, who is not involved in the Iranian man's care. These tumors contain tissue normally found in another part of the body. Most frequently, limbal dermoids contain hair follicles, but they can also contain other tissues, including cartilage and sweat glands, Fromer said.

These tumors can cause astigmatism (blurred vision), but usually don't cause dramatic vision problems, Fromer said. That's because they typically do not cover the center of the cornea, an important part of the eye for vision, he said.